Distance between pairs of coordinastes (UTM) in km with R-CodePudding

I have a dataframe with 2 pairs of UTM (32N) coordinates and I need to compute the differences in km between each of them, from origin to destination. I'm trying with sf library, using "by_element" but I obtain an error message "Error in st_distance(data, by_element = TRUE) : !missing_y non è TRUE". What's wrong? If I use it without the "by_element" option, it works and the distance matrix between all coordinates is created, but this is not what I need.

library(sf)
df <- data.frame(id = c(1,2,3), x_origin = c(642683.2, 373775,383881 ), y_origin = c(5082920, 4997274,4994504), x_dest =c(642683.3, 1126050,942763.9 ), y_dest=c(5082920, 4374481,4534235  )) 

data <- st_as_sf(df, coords = c("x_origin", "y_origin"), crs="4326" )

distances <- st_distance(data, by_element = TRUE  )

CodePudding user response：

You have provided x (origin) to st_distance()but no y (destination). And that CRS can't be right, sf doesn't recognise EPSG code if it's a string and 4326 would suggest coordinates are in WGS84 lat/long. Assuming 32632, WGS 84 / UTM zone 32N.

library(sf)
df <- data.frame(id = c(1,2,3), x_origin = c(642683.2, 373775,383881 ), y_origin = c(5082920, 4997274,4994504), x_dest =c(642683.3, 1126050,942763.9 ), y_dest=c(5082920, 4374481,4534235  )) 
origin <- st_as_sf(df, coords = c("x_origin", "y_origin"), crs=32632 )
dest <- st_as_sf(df, coords = c("x_dest", "y_dest"), crs=32632 )

(distances <- st_distance(origin, dest,  by_element = TRUE ))
#> Units: [m]
#>        1        2        3 
#>      0.1 976621.1 724015.0

^{Created on 2023-01-25 with reprex v2.0.2}

CodePudding user response：

First, sf can't know that you are calculating distances between origin and destination by the way you have input the data. Second, the EPSG code for UTM Zone 32N is not 4326. Third, you should have used crs = st_crs(4326) instead of crs = "4326".

Use the following piece of code to create the objects needed to calculate the distances you are interested in

library(sf)
df <- data.frame(id = c(1, 2, 3),
                 x_origin = c(642683.2, 373775, 383881),
                 y_origin = c(5082920, 4997274, 4994504),
                 x_dest = c(642683.3, 1126050, 942763.9),
                 y_dest = c(5082920, 4374481, 4534235))

origin <- st_as_sf(df, coords = c("x_origin", "y_origin"),
                   crs = st_crs(32632))
dest <- st_as_sf(df, coords = c("x_dest", "y_dest"),
                 crs = st_crs(32632))

Note the different EPSG code for the CRS.

Next, we calculate the distances (the default of this projection is in meters)

distances <- st_distance(origin, dest, by_element = TRUE)

If you use by_element = FALSE, you get all the pairwise distances.

Lastly, we can use the package units to convert the distances to km (or we can simply divide them by 1000).

units::set_units(distances, "km")
> Units: [km]
> [1]   0.0001 976.6211 724.0150